<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>

        .box{
            /* width: 100px; */
            width: 0;
            /* height: 100px; */
            
            border: 100px solid;
            border-color: red transparent transparent transparent;

            border-width: 400px 100px;

        }
    </style>
</head>
<body>
    <!-- 利用 border 可以制作三角形 -->

    <div class="box">

    </div>
</body>
</html>